With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Also, in consideration of global environments, electric cars and hybrid cars have been recently developed and put into practice, so that there is an increasing demand for lithium ion secondary batteries used in large size applications which have excellent storage characteristics. Under these circumstances, the lithium ion secondary batteries having advantages such as large charge/discharge capacities have been noticed.
Hitherto, as positive electrode active substances useful for high energy-type lithium ion secondary batteries exhibiting a 4 v-grade voltage, there are generally known LiMn2O4 having a spinel structure, LiMnO2 having a zigzag layer structure, LiCoO2 and LiNiO2 having a layer rock-salt structure, or the like. Among the secondary batteries using these active substances, lithium ion secondary batteries using LiNiO2 have been noticed because of large charge/discharge capacities thereof. However, in the case where the secondary batteries are used in a movable body requiring a large energy such as electric cars, these materials tend to be still insufficient in discharge capacity, and, therefore, it has been required to further improve properties thereof.
In recent years, it has been found that a positive electrode active substance comprising Li2MnO3 belonging to a space group of C2/m and having a higher capacity exhibits a large discharge capacity (Patent Literature 1).
With respect to the above material, there have also been made various studies on enhancement of a discharge capacity and a rate characteristic thereof. It has been reported that the properties of the material is improved by additives (Patent Literature 2), a heat treatment method (Patent Literature 3), a method of trapping gases generated upon charging (Patent Literature 4), a method of modifying properties of the surface of the particles (Patent Literature 5), etc. However, these conventional methods tend to be still insufficient.